Active material for battery and method of preparing the same

ABSTRACT

An active material for a battery has a surface-treatment layer including a compound of the formula (1):  
     MXO k   (1)  
     wherein M is at least one element selected from an alkali metal, an alkaline earth metal, a group 13 element, a group 14 element, a transition metal, and a rare-earth element, X is an element capable of forming a double bond with oxygen, and k is a numerical value in the range of 2 to 4.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit under 35 U.S.C. §119(e) ofthe U.S. Provisional Application Serial No. 60/297,783, entitled “ACTIVEMATERIAL FOR BATTERY AND METHOD FOR PREPARING SAME”, filed Jun. 14,2001, and No. 60/304,793, of the same title, filed Jul. 13, 2001.

BACKGROUND OF THE INVENTION

[0002] (a) Field of the Invention

[0003] The present invention relates to an active material for a batteryand a method of preparing the same, and more specifically to an activematerial for a battery with excellent electrochemical characteristicsand thermal stability, and a method of preparing the same.

[0004] (b) Description of the Related Art

[0005] Recently, in relation to trends toward more compact and lighterportable electronic equipment, there has been a growing need to developa high performance and large capacity battery to be used for electricpower for portable electronic equipment. Also, there has been extensiveresearch on batteries with good safety characteristics and low cost.

[0006] Generally, batteries are classified as primary batteries that canbe used only once and secondary batteries that are rechargeable. Primarybatteries include manganese batteries, alkaline batteries, mercurybatteries, silver oxide batteries and so on, and secondary batteriesinclude lead-acid storage batteries, Ni—MH (nickel metal hydride)batteries, nickel-cadmium batteries, lithium metal batteries, lithiumion batteries, lithium polymer batteries and lithium-sulfur batteries.

[0007] These batteries generate electric power by using materialscapable of electrochemical reactions at positive and negativeelectrodes. Factors that affect battery performance characteristics suchas capacity, cycle life, power capability, safety and reliability,include electrochemical properties and thermal stability of activematerials that participate in electrochemical reactions at the positiveand negative electrodes. Therefore, research to improve theelectrochemical properties and thermal stability of the active materialsat the positive and negative electrodes continues.

[0008] Among the active materials currently being used for negativeelectrodes of batteries, lithium metal provides both high capacitybecause it has a high electric capacity per unit mass and high voltagedue to a relatively high electronegativity. However, since it isdifficult to assure the safety of a battery using lithium metal, othermaterials that can reversibly deintercalate and intercalate lithium ionsare being used extensively for the active material of the negativeelectrodes in lithium secondary batteries.

[0009] Lithium secondary batteries use materials that reversiblyintercalate or deintercalate lithium ions during charge and dischargereactions for both positive and negative active materials, and containorganic electrolyte or polymer electrolyte between the positiveelectrode and the negative electrode. This battery generates electricenergy from changes of chemical potential during theintercalation/deintercalation of lithium ions at the positive andnegative electrodes.

[0010] Lithium metal compounds of a complex formula are used as thepositive active material of the lithium secondary battery. Typicalexamples include LiCoO₂, LiMn₂O₄, LiNiO₂, LiNi_(1-x)Co_(x)O₂ (0<x<1),LiMnO₂ and a mixture of these compounds. Manganese-based positive activematerials such as LiMn₂O₄ or LiMnO₂ are the easiest to synthesize, lesscostly than the other materials, and environmentally friendly. However,these manganese-based materials have relatively low capacity. LiCoO₂ hasgood electric conductivity, high battery voltage and excellent electrodecharacteristics. This compound is presently the most popular materialfor positive electrodes of commercially available Li-ion batteries.However, it is relatively expensive and has low stability duringcharge-discharge at a high rate. LiNiO₂ is currently the least costly ofthe positive active materials mentioned above and has a high dischargecapacity, but it is difficult to synthesize and the least stable amongthe compounds mentioned above.

[0011] The above active materials are lithiated intercalation compoundsin which stability and capacity of active material is determined by thenature of intercalation/deintercalation reactions of lithium ions. Asthe charging potential increases, the amount of Li deintercalationincreases, thus increasing the electrode capacity, but thermal stabilityof the electrode decreases steeply due to its structural instability.When the interior temperature of the battery increases in the fullycharged state, the bonding energy between the metal ions and the oxygenof the active material decreases, releasing oxygen when a temperatureabove a threshold value is reached. For example, LiCoO₂ active materialin a charged state has the formula Li_(1-x)CoO₂, where O<x<1. Becausethe active material having the above structural formula is unstable,especially when x>0.5, if the interior temperature of the batteryincreases beyond the threshold value, oxygen gas (O₂) is released. Sincethe reaction of this oxygen with organic electrolyte in the battery ishighly exothermic, a thermal runaway situation may be created in thebattery, and this may cause an explosion in the battery. Therefore, itis desirable to control the threshold temperature and the amount ofexothermic heat evolved from the reaction in order to improve the safetyof the battery.

[0012] One way of controlling the threshold temperature and the amountof exothermic heat is controlling the surface area of the activematerial through particle size control, which is usually achieved bypulverizing and sieving the active material. The smaller the particlesize, i.e. the larger the surface area, the better the batteryperformance, in particular the power capability, i.e. capacity valuesand discharge voltages at low temperatures and at high rates. However,battery safety, cycle life and self-discharge become worse as theparticle size decreases. Because of these conflicting factors, there isa practical limitation in controlling the threshold temperature and heatevolution rate through particle size alone.

[0013] In order to improve stability of active material itself duringcharge-discharge, it has been suggested to dope other elements into theNi-based or Co-based lithium oxide. For example, U.S. Pat. No. 5,292,601discloses Li_(x)MO₂ (where M is at least one element selected from Co,Ni and Mn; and x is 0.5 to 1) as an improved material over LiCoO₂.

[0014] Another attempt to improve stability includes modifying thesurface of the active material. Japanese Patent Laid-Open No. Hei9-55210 discloses that lithium nickel-based oxide is coated withalkoxide of Co, Al and Mn and is heat-treated to prepare a positiveactive material. Japanese Patent Laid-Open No. Hei 11-16566 discloseslithium-based oxide coated with a metal and/or an oxide thereof. Themetal includes Ti, Sn, Bi, Cu, Si, Ga, W, Zr, B or Mo. Japanese PatentLaid-Open No. Hei 11-185758 discloses coating a surface of lithiummanganese oxide with a metal oxide by using a co-precipitation processand heat-treating the same to prepare a positive active material.

[0015] However, the above methods did not solve the fundamental problemsassociated with the safety of the battery: The threshold temperaturewherein the active material prepared according to the above methodsbegins to react with an electrolyte, that is, the decompositiontemperature, at which oxygen bound to metal of the active materialbegins to be released (exothermic starting temperature, T_(s)) does notshift sufficiently to a higher temperature and the amount of releasedoxygen (the value related to the exothermic heat) does not decreasesufficiently by the methods described above.

[0016] The structural stability of positive active material having thecomposition of Li_(1-x)MO₂ (M=Ni or Co) during charging is stronglyinfluenced by the value of x. That is, when 0<x<0.5, cyclic stability issteadily and stably maintained, but when x is greater than or equal to0.5, phase transition occurs from a hexagonal phase to a monoclinicphase. This phase transition causes an anisotropic volume change, whichinduces development of micro-cracks in the positive active material.These micro-cracks damage the structure of the active material, and thusthe battery capacity decreases dramatically and the cycle life isreduced. Therefore, when anisotropic volume change is minimized, thecapacity and the cycle life of the battery are improved.

[0017] In order to increase structural stability of positive activematerial, U.S. Pat. No. 5,705,291 discloses a method in which acomposition comprising borate, aluminate, silicate or mixtures thereofwas coated onto the surface of a lithiated intercalation compound, butit still has a problem with structural stability.

[0018] In the above description, positive active materials of lithiumsecondary batteries and related examples of developments were explained.Recently, in relation to the tendency to develop portable electronicequipment that is more compact and lightweight, other types of batterieshave the same demands for an active material that guarantees batteryperformance, safety and reliability. Research and development istherefore accelerated on electrochemical properties and thermalstability of positive active materials to ensure improved performance,safety and reliability of batteries.

SUMMARY OF THE INVENTION

[0019] In order to solve the problems stated above, it is an object ofthe present invention to provide an active material for a battery withgood electrochemical characteristics, such as capacity and cycle life.

[0020] It is another object to provide an active material for a batterywith good thermal stability.

[0021] It is still another object to provide a method of preparing anactive material with good manufacturing productivity and an economicalpreparation process.

[0022] In order to accomplish these and other objects, the presentinvention provides an active material for a battery having a surfacetreatment layer comprising the compound having the formula (1):

MXO_(k)  (1)

[0023] wherein M is at least one selected from the group consisting ofan alkali metal, an alkaline earth metal, a group 13 element, a group 14element, a transition metal and a rare-earth element; X is an elementthat can form a double bond with oxygen; and k is a numerical value inthe range of 2 to 4.

[0024] The present invention also provides a process for preparing anactive material for a battery comprising: preparing a coating liquid byadding a compound comprising an element X that is capable of forming adouble bond with oxygen, and a compound comprising at least one from thegroup consisting of an alkali metal, an alkaline earth metal, a group 13element, a group 14 element, a transition metal, and a rare-earthelement, to water; adding active material to the coating liquid to coatthe material with the suspension; and heat-treating the coated activematerial to form a surface-treatment layer comprising the compound ofthe formula (1).

[0025] Other features and advantages of the present invention will beapparent from the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention.

[0027]FIG. 1 shows voltage and capacity characteristics at variousC-rate discharges of half-cells of Example 1 of the present inventionand Comparative Example 1.

[0028]FIG. 2 shows voltage and capacity characteristics of half-cellsaccording to Example 1 of the present invention and Comparative Example1 at various cycle numbers at a 1C rate of discharge.

[0029]FIG. 3A shows charge and discharge curves of a half-cell accordingto Example 5 of the present invention at rates of 0.2C, 0.5C, and 1C inthe voltage range of 4.3V to 2.75V.

[0030]FIG. 3B is an expanded view of a portion of FIG. 3A.

[0031]FIG. 4 shows charge and discharge curves of a half-cell accordingto Example 2 of the present invention at rates of 0.2C, 0.5C and 1C inthe voltage range of 4.3V to 2.75V.

[0032]FIG. 5 shows charge and discharge curves of a half-cell accordingto Example 12 of the present invention at rates of 0.2C, 0.5C and 1C inthe voltage range of 4.3V to 2.75V.

[0033]FIG. 6 shows voltage and discharge capacities of half-cellsaccording to Examples 5 to 7 of the present invention and ComparativeExample 1 at 1 C rate of discharge.

[0034]FIG. 7 shows charge and discharge curves of half-cells accordingto Example 1 of the present invention and Comparative Example 1 at ratesof 0.2C, 0.5C and 1C in the voltage range of 4.6V to 2.75V.

[0035]FIG. 8 shows discharge curves of a half-cell according to Example1 of the present invention at rates of 0.2C, 0.5C and 1 C in the voltagerange of 4.95V to 2.75V.

[0036]FIG. 9 shows cycle life characteristics at 1C rate of half-cellsof Example 1 of the present invention and Comparative Example 3.

[0037]FIG. 10 shows discharge curves of a Li-ion cell comprising activematerial of Example 15 of the present invention at rates of 0.5C, 1 Cand 2C in the voltage range of 4.2V to 2.75V.

[0038]FIGS. 11A to 11E show the results of elemental analyses of activematerials prepared according to Example 1 of the present invention usingScanning Transmission Electron Microscopy (STEM).

[0039]FIG. 12 shows the results of elemental analyses for components ofa surface-treatment layer of active material prepared according toExample 1 of the present invention using Auger Spectroscopy.

[0040]FIGS. 13A and 13B show the results of elemental distributionanalyses for a cross-sectional view of active material preparedaccording to Example 1 of the present invention, by line scanning withElectron Probe Micro Analysis (EPMA).

[0041]FIG. 14 shows the results of an analysis of a surface-treatmentlayer of active material of Example 1 of the present invention, by X-rayphotoelectron spectroscopy (XPS).

[0042]FIGS. 15A and 15B show cyclic voltammograms of active materialsaccording to Example 5 of the present invention and Comparative Example1.

[0043]FIG. 16 shows diffusion coefficients (D_(Li+)) of lithium ions ofactive materials of Example 5 according to the present invention andComparative Example 1.

[0044]FIGS. 17A and 17B show changes of lattice constants of the activematerial in various states of charge of half-cells prepared according toExample 5 of the present invention and Comparative Example 1, in thevoltage range of 4.6V to 4.25V.

[0045]FIG. 18 shows the results of Differential Scanning Calorimetry(DSC) of active materials obtained after charging half-cells preparedaccording to Example 5 of the present invention and Comparative Example1, at 4.3V.

[0046]FIG. 19 shows the results of Differential Scanning Calorimetry(DSC) of active materials obtained after charging half-cells preparedaccording to Examples 5 to 7 of the present invention and ComparativeExample 1, at 4.3V.

[0047]FIG. 20 shows the results of Differential Scanning Calorimetry(DSC) of active materials obtained after overcharging half-cellsprepared according to Example 1 of the present invention and ComparativeExample 1, at 4.6V.

[0048]FIG. 21 shows the results of Differential Scanning Calorimetry(DSC) of active materials obtained after overcharging half-cellsprepared according to Example 8 of the present invention and ComparativeExample 2, at 4.6V.

[0049]FIG. 22 shows the results of Differential Scanning Calorimetry(DSC) of active materials obtained after overcharging half-cellsprepared according to Example 8 of the present invention and ComparativeExamples 2 and 4, at 5V.

[0050]FIG. 23 shows the results of Differential Scanning Calorimetry(DSC) of active materials obtained after overcharging half-cellsprepared according to Examples 1 and 9 of the present invention andComparative Examples 1 and 4, at 5V.

[0051]FIG. 24 shows the results of Differential Scanning Calorimetry(DSC) of active materials obtained after overcharging half-cellsprepared according to Example 10 of the present invention andComparative Example 4, at 5V.

[0052]FIG. 25 shows the results of Differential Scanning Calorimetry(DSC) of active materials obtained after overcharging half-cellsprepared according to Examples 12 and 13 of the present invention andComparative Example 5, at 4.6V.

[0053]FIG. 26 shows charge voltage and cell temperature of a Li-ion cellcomprising active material prepared in Example 1 when the cell isovercharged at 1C rate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0054] The active material for a battery of the present inventioncomprises a surface-treatment layer comprising a compound with theformula (1) on the surface thereof:

MXO_(k)  (1)

[0055] wherein M is at least one element selected from the groupconsisting of an alkali metal, an alkaline earth metal, a group 13element, a group 14 element, a transition metal, and a rare-earthelement; X is an element that is capable of forming a double bond withoxygen; and k is a numerical value in the range of 2 to 4.

[0056] The group 13 element (according to the new IUPAC agreement)refers to the element group including Al of the Periodic Table. Thegroup 14 element (according to the new IUPAC agreement) refers to theelement group including Si of the Periodic Table. In the preferredexamples of the present invention, M includes Na, K, Mg, Ca, Sr, Ni, Co,Si, Ti, B, Al, Sn, Mn, Cr, Fe, V, Zr, or a combination thereof, and X isP, S, W or a combination thereof. The element X forms a double bond withoxygen, which means a classical chemical bonding. For example, inclassical Chemistry, when X bonds with four oxygen elements, it meansone double bond and three single bonds. However, in modern Chemistry, itmeans that X bonds with 1.25 oxygens because of delocalization ofelectrons.

[0057] The amount of element M of the compound with the formula (1) ofthe present invention is 0.1 to 15% by weight, preferably 0.1 to 6% byweight of the active material. Also, the amount of element X that iscapable of forming a double bond with oxygen of the compound having theformula (1) is 0.1 to 15% by weight, preferably 0.1 to 6% by weight ofthe active material. When the amount of M or X present in the surface ofthe active material is not in the above range, electrochemicalcharacteristics at a high rate are not improved and the thermalstability is not improved by the coating.

[0058] The thickness of the surface-treatment layer of the presentinvention is preferably 0.01 to 2 μm, and more preferably 0.01 to 1 μm.While other thicknesses are possible, if the thickness of thesurface-treatment layer is less than 0.01 μm, the effect obtained fromthe surface-treatment layer may not be realized. In contrast, if thethickness is more than 2 μm, the capacity of the battery isdeteriorated.

[0059] In the case that the surface-treated active material is alithiated intercalation compound, a solid-solution compound between thelithiated intercalation compound and the MXO_(k) compound with theformula (1) is formed on the surface of the active material in additionto the MXO_(k) compound. In this case, a surface-treatment layer of theactive material comprises both the solid-solution compound and theMXO_(k) compound. The solid-solution compound comprises Li, M′ (M′ is atleast one selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe,Mg, Sr, V, and a rare-earth element, that originate from the lithiatedintercalation compound), M (M is at least one selected from the groupconsisting of an alkali metal, an alkaline earth metal, a group 13element, a group 14 element, a transition metal, and a rare-earthelement), X (an element capable of forming a double bond with oxygen),and O (oxygen).

[0060] When the surface-treatment layer comprising the solid-solutioncompound and the MXO_(k) compound on the surface of these intercalationcompounds is formed, the elements M and X have a concentration gradientfrom the surface of the active material toward the center of the activematerial particle grain. That is, M and X have a high concentration atthe surface of the active material and the concentration graduallydecreases toward the inside of the particle.

[0061] According to the preferable examples of the present invention,the active material for a battery comprising a lithiated intercalationcompound and a surface-treatment layer comprising the solid-solutioncompound with Al and P, and AlPO_(k) (k is 2 to 4) is provided.

[0062] The surface treatment technique of the active material with theMXO_(k) compound of the present invention may be used for all batteries,and is effective in improving the performance characteristics of bothactive materials for the positive electrodes as well as the negativeelectrodes. The surface-treated active material includes materials thatcan undergo reversible electrochemical oxidation-reduction reactions.The electrochemically oxidizable and reducible material includes ametal, a lithium-containing alloy, sulfur-based compounds, compoundsthat can reversibly form lithium-containing compounds by a reaction withlithium ions, all materials that can reversiblyintercalate/deintercalate lithium ions (lithiated intercalationcompounds), although the present invention is not limited thereto.

[0063] The metal includes lithium, tin or titanium. Thelithium-containing alloy includes a lithium/aluminum alloy, alithium/tin alloy, or a lithium/magnesium alloy. The sulfur-basedcompound which is the positive active material of the lithium-sulfurbattery includes a sulfur element, Li₂S_(n) (n≧1), an organic sulfurcompound and a carbon-sulfur polymer ((C₂S_(x))_(n) where x=2.5 to 50and n≧2). The compound that can reversibly form a lithium-containingcompound by a reaction with lithium ions includes silicon, titaniumnitrate or tin oxide (SnO₂).

[0064] The active material that can reversibly intercalate/deintercalatelithium ion (lithiated intercalation compounds) includes carbon-basedmaterial, lithium-containing metal oxides, and lithium-containingchalcogenide compounds. The carbon-based material can be non-crystallinecarbon, crystalline carbon, or a mixture thereof. Examples of thenon-crystalline carbon includes soft carbon (low temperature calcinatedcarbon), and hard carbon (high temperature calcinated carbon). Examplesof crystalline carbon include natural graphite or artificial graphitewhich are plate, sphere or fiber shape.

[0065] The lithium-containing metal oxide and lithium-containingchalcogenide compound has a monoclinic, hexagonal or cubic structure asa basic structure.

[0066] A conventional lithium-containing compound (lithium-containingmetal oxide and lithium-containing chalcogenide compound) can be used asthe lithiated intercalation compound of the present invention, andpreferable examples are as follows: Li_(x)Mn_(1−y)M′_(y)A₂ (2)Li_(x)Mn_(1−y)M′_(y)O_(2−z)B_(z) (3) Li_(x)Mn₂O_(4−z)B_(z) (4)Li_(x)Mn_(2−y)M′_(y)A₄ (5) Li_(x)Co_(1−y)M′_(y)A₂ (6)Li_(x)Co_(1−y)M′_(y)O_(2−z)B_(z) (7) Li_(x)Ni_(1−y)M′_(y)A₂ (8)Li_(x)Ni_(1−y)M′_(y)O_(2−z)B_(z) (9) Li_(x)Ni_(1−y)Co_(y)O_(2−z)B_(z)(10) Li_(x)Ni_(1−y−z)Co_(y)M′_(z)A_(a) (11)Li_(x)Ni_(1−y−z)Co_(y)M′_(z)O_(2−a)B_(a) (12)Li_(x)Ni_(1−y−z)Mn_(y)M′_(z)A_(a) (13)Li_(x)Ni_(1−y−z)Mn_(y)M′_(z)O_(2−a)B_(a) (14)

[0067] wherein 0.95≦x≦1.1,0≦y≦0.5,0>z>0.5, and 0<α≦2;

[0068] M′ is at least one element selected from the group consisting ofAl, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, and a rare-earth element;

[0069] A is at least one element selected from the group consisting ofO, F, S and P; and

[0070] B is at least one element selected from the group consisting ofF, S and P.

[0071] The average particle size of these lithiated intercalationcompounds is preferably 1 to 20 μm, more preferably 3 to 15 μm.

[0072] In the present invention, a surface-treatment layer comprisingthe MXO_(k) compound is formed on the surface of the active material.When the active material is a lithiated intercalation compound, asurface-treatment layer comprising a solid-solution compound includingLi, M′ (M′ is at least one selected from the group consisting of Al, Ni,Co, Mn, Cr, Fe, Mg, Sr, V, and a rare-earth element that originate fromlithiated intercalation compounds), M (M is at least one selected fromthe group consisting of an alkali metal, an alkaline earth metal, agroup 13 element, a group 14 element, a transition metal, and arare-earth element), X (an element capable of forming a double bond withoxygen) and O; as well as the MXO_(k) compound, is formed.

[0073] As a general rule, the capacity of a battery cell using theactive material with a high tap density is greater than one using asimilar active material having a low tap density. Therefore, an improvedtap density of the active material is generally desired for a batterycell. The active materials surface-treated according to the presentinvention show a much higher tap density than the correspondingequivalent active material without surface-treatment, indicating thatthe surface-treatment facilitates compaction of the active materialpowder. The tap density of the active material of the present inventionis maintained at 1 to 3 g/cc, thus it increases the capacity of thebattery cell. According to the preferable example of the presentinvention, the tap density of the active material is more than about 2g/cc.

[0074] The active materials, with the surface-treated according to thepresent invention, also produces electrodes having a much higherelectrode density, meaning more active material per unit volume, thanthe corresponding active material without surface-treatment, when theelectrodes are fabricated by a conventional electrode fabricationprocess used by the Li-ion battery industries which involves a coatingonto a current collector of an active material slurry comprising aconductive agent, a binder and a solvent in addition to the activematerial. When the electrodes are compacted by compression, theelectrodes containing the surface-treated active material compact wellwithout micro-cracks in the active material powder, while the electrodecontaining bare active material shows micro-cracks in the activematerial powder. The surface-treatment of the active material of thepresent invention might possibly provide a lubricating effect on thesurface of the powder for improved compaction of the active material.

[0075] The most important factor affecting safety of a battery is thereactivity of the active material in a charged state at its surfacetoward the electrolyte. For example, one of lithiated intercalationcompounds, LiCoO₂, has a structure of α-NaFeO₂, while it has a structureof Li_(1-x)CoO₂ during charge and discharge cycles. When it is chargedat a voltage over 4.93V, Li is completely removed, and it has astructure of a hexagonal type of Cdl₂. In such a lithium metal oxide, asthe amount of lithium decreases, thermal stability decreases and itbecomes a stronger oxidant. When a battery containing LiCoO₂ activematerial is fully charged, the active material composition becomesLi_(1-x)CoO₂ where x is greater than or equal to 0.5. Such a compositionbecomes unstable as the battery temperature rises, i.e., the oxygenbound with metal, that is cobalt, is released to form gaseous O₂. Thereleased oxygen might react with electrolyte inside the battery,possibly leading to an explosion. Therefore, the oxygen-releasingtemperature (exothermic reaction starting temperature) and the amount ofexothermic heat released by the reaction are important factors todetermine the safety of the battery. Such thermal stability can beevaluated from DSC (Differential Scanning Calorimetry) curves bydetermining the starting temperature of the exothermic reaction and theheat of reaction.

[0076] Unlike conventional active material, for the active material thatis surface-treated with the MXO_(k) compound of the present invention,the DSC exothermic peak is almost negligible in size since the MXO_(k)compound inhibits reaction with electrolyte. Therefore, the activematerial of the present invention is substantially improved in thermalstability over the conventional surface-untreated active material.

[0077] The surface-treatment layer comprising the MXO_(k) compound ofthe present invention may be applied to the active material for aprimary battery such as a manganese battery, an alkaline battery, amercury battery, a silver oxide battery, as well as to the activematerial for a secondary battery such as a lead-acid storage battery, aNi—MH (nickel metal hydride) battery, a nickel-cadmium battery, alithium metal battery, a lithium ion battery, a lithium polymer batteryand a lithium-sulfur battery. The structures of such batteries,including a lithium secondary battery shell, are well known, asindicated, e.g., by U.S. Pat. No. 5,753,387, the disclosure of which isincorporated by reference herein. The active material having thesurface-treatment layer is used in at least one of a positive electrodeand a negative electrode of the above batteries.

[0078] The process for preparing active material having thesurface-treatment layer of MXO_(k) compound is as follows.

[0079] First, the coating liquid is prepared by reacting a compoundcomprising X (an element that is capable of forming a double bond withoxygen) with a compound comprising M (an alkali metal, an alkaline earthmetal, a group 13 element, a group 14 element, a transition metal, arare-earth element or a combination thereof) in water. In thisinvention, “coating liquid” refers to a homogeneous suspension or asolution.

[0080] Since water is used as a solvent in the coating liquid, thepresent process is advantageous over the process using an organicsolvent for the process cost-reduction.

[0081] The choice of the compound type comprising element (X) has noparticular limitation as long as the compound is soluble in water. Forexample, when X is P, it can be diammonium hydrogen phosphate((NH₄)₂HPO₄), P₂O₅, H₃PO₄, or Li₃PO₄. The content of the compoundcomprising X is preferably 0.01 to 30% by weight, more preferably 0.1 to20% by weight of the total weight of the coating liquid.

[0082] The element (M) used for the coating liquid is an alkali metal,an alkaline earth metal, a group 13 element, a group 14 element, atransition metal, a rare-earth element or a combination thereof. Thepreferable examples of these elements are Al, Ni, Co, Zr, Mn, Cr, Fe,Mg, Sr, V, or a combination thereof. The choice of the compound typecomprising these elements has no particular limitation as long as thecompound is soluble in water. The preferred examples are a nitrate andan acetate. The amount of the compound comprising an alkali metal, analkaline earth metal, a group 13 element, a group 14 element, atransition metal, a rare-earth element or a combination thereof ispreferably 0.01 to 30% by weight, more preferably 0.1 to 20% by weightof the weight of the coating liquid.

[0083] The coating liquid as prepared above is used to coat the activematerial. The coating may be achieved by simply adding a predeterminedamount of the coating liquid to a given amount of the active materialpowder followed by a through mixing and optionally drying, although thepresent invention is not limited to this method.

[0084] Then, the coated active material is heat-treated at 100 to 700°C., preferably at 100 to 500° C. for 1 to 20 hours. If theheat-treatment process is over-done, an AlPO_(k) (k is 2 to 4) compounddiffuses into the inside of the active material resulting in a batterycapacity decrease. Before the heat-treatment process, a separate dryingprocess that dries the coated liquid may be used. In the presentinvention, since the heat-treatment process is made at a lowertemperature and for a shorter time than a prior-art process usingorganic solvent, which requires a higher calcination temperature and alonger calcination time, it reduces cost during large-scale production.

[0085] In the prior-art process, a sieving process step is requiredsince particle agglomerations occur usually due to a high calcinationtemperature. However, in the process of the present invention, such asieving process is not required since the calcination temperature issignificantly reduced resulting in negligible particle agglomerations.

[0086] The desired surface-treatment layer comprising the MXO_(k)compound is formed on the surface of the active material after theheat-treatment process. When the active material being coated is alithiated intercalation compound, a solid-solution compound which isformed by combination of the lithiated intercalation compound and theMXO_(k) compound may be formed between the layer of MXO_(k) compound andthe active material.

[0087] In forming the battery, the method includes preparing a coatingliquid by adding a compound including an element that is capable offorming a double bond with oxygen of lithium metal oxide, and a metalcompound comprising at least one element from the group consisting of analkali metal, an alkaline earth metal, a group 13 element, a group 14element, a transition metal, and a rare-earth element, to water. Anactive material is added to the coating liquid to coat the activematerial. The coated active material is heat treated to prepare anactive material having a surface-treatment layer comprising a compoundhaving the formula (1):

MXO_(k)  (1)

[0088] wherein M is at least one element selected from the groupconsisting of an alkali metal, an alkaline earth metal, a group 13element, a group 14 element, a transition metal, and a rare-earthelement, X is an element that is capable of forming a double bond withoxygen, and k is a numerical value in the range of 2 to 4. Next, aslurry comprising the active material with the surface treatment layeris coated onto a current collector to prepare at least one of a positiveand negative electrode which is used in the fabrication of a battery.

[0089] The present invention is further explained in more detail withreference to the following examples. These examples, however, should notin any sense be interpreted as limiting the scope of the presentinvention.

EXAMPLE 1

[0090] A coating liquid was prepared by adding 1 g of (NH₄)₂HPO₄ and 1.5g of aluminum nitrate (Al(NO₃)₃·9H₂O) to 100 ml of water. The resultingliquid was a homogeneous colloidal suspension of amorphous AlPO_(k)phase. After adding a 10 ml portion of the coating liquid to 20g ofLiCoO₂ powder having an average particle diameter of 10 μm, it wasthoroughly mixed before drying at 130° C. for 30 minutes. The positiveactive material with the coating layer comprising a solid-solutioncompound including Al and P and the AlPO_(k) compound on the surface wasfurther heat-treated at 400° C. for 5 hours to obtain the desiredcoating. The total amount of Al and P was 1% by weight of the totalactive material weight.

[0091] The slurry for the positive electrode containing the positiveactive material as described, super P (conductive agent), andpolyvinylidene fluoride (binder) in the weight ratio of 94/3/3 wasprepared by mixing them thoroughly in an N-methyl pyrrolidone (NMP)solvent. The slurry composition comprising the positive active materialwas coated on an Al foil at a thickness of about 300 μm, dried for 20minutes at 130° C., and pressed under a 1 ton pressure to make apositive electrode for a coin cell. A coin-typed half-cell was preparedby using this positive electrode and lithium metal as a counterelectrode. For the electrolyte, 1 M LiPF₆ solution of mixed solvent ofethylene carbonate (EC) and dimethyl carbonate (DMC) in the volume ratioof 1:1 was used.

EXAMPLE 2

[0092] A coin-typed half-cell was prepared by the same method as inExample 1, except that a 15 ml portion of the coating liquid of Example1 was added to 20 g of LiCoO₂ with an average particle diameter of 10μm. The total amount of Al and P was 1.5% by weight of the total activematerial weight.

EXAMPLE 3

[0093] A coin-typed half-cell was prepared by the same method as inExample 1, except that a 20 ml portion of the coating liquid of Example1 was added to 20 g of LiCoO₂ with an average particle diameter of 10μm. The total amount of Al and P was 2% by weight of the total activematerial weight.

EXAMPLE 4

[0094] A coin-typed half-cell was prepared by the same method as inExample 1, except that a 10 ml portion of the coating liquid of Example1 was added to 20 g of LiCoO₂ with an average particle diameter of 5 μm.The total amount of Al and P was 1% by weight of the total activematerial weight.

EXAMPLE 5

[0095] A coin-typed half-cell was prepared by the same method as inExample 1, except that the heat-treatment time was 10 hours.

EXAMPLE 6

[0096] A coin-typed half-cell was prepared by the same method as inExample 1, except that the heat-treatment step was made at 500° C. for 5hours.

EXAMPLE 7

[0097] A coin-typed half-cell was prepared by the same method as inExample 1, except that the heat-treatment step was made at 500° C. for10 hours.

EXAMPLE 8

[0098] A coin-typed half-cell was prepared by the same method as inExample 1, except that a 20 ml portion of the coating liquid of Example1 was added to 20 g of LiCoO₂ with an average particle diameter of 5 μm,and the heat-treatment step was made at 400° C. for 10 hours. The totalamount of Al and P was 2% by weight of the total active material weight.

EXAMPLE 9

[0099] A coin-typed half-cell was prepared by the same method as inExample 1, except that a 15 ml portion of the coating liquid of Example1 was added to 20 g of LiCoO₂ with an average particle diameter of 10μm, and the heat-treatment step was made at 400° C. for 10 hours. Thetotal amount of Al and P was 1.5% by weight of the total active materialweight.

EXAMPLE 10

[0100] A coin-typed half-cell was prepared by the same method as inExample 1, except that LiMn₂O₄ with an average particle diameter of 13μm was used instead of LiCoO₂.

EXAMPLE 11

[0101] A coin-typed half-cell was prepared by the same method as inExample 1, except that LiNi_(0.9)Co_(0.1)Sr_(0.002)O₂with an averageparticle diameter of 13 μm was used instead of LiCoO₂.

EXAMPLE 12

[0102] A coin-typed half-cell was prepared by the same method as inExample 1, except that LiNi_(0.8)Mn_(0.2)O₂with an average particlediameter of 10 μm was used instead of LiCoO₂ and the heat-treatment stepwas made at 400° C. for 10 hours.

EXAMPLE 13

[0103] A coin-typed half-cell was prepared by the same method as inExample 1, except that 20 g of LiNi_(0.8)Mn_(0.2)O₂with an averagediameter of 10 μm was used instead of LiCoO₂, it was coated with 20 mlof coating liquid prepared in Example 1 and the heat-treatment step wasmade at 400° C. for 10 hours.

EXAMPLE 14

[0104] A coin-typed half-cell was prepared by the same method as inExample 1, except that Li_(1.03)Ni_(0.69)Mn_(0.19)Co_(0.1)Al_(0.07)Mg_(0.07)O₂ with an average diameter of 13 μmwas used instead of LiCoO₂.

EXAMPLE 15

[0105] A slurry containing positive active material was prepared bymixing the positive active material of Example 1, super P (conductiveagent), and polyvinylidene fluoride (binder) in the weight ratio of96/2/2 in a mixing solvent of N-methyl pyrrolidone (NMP). The positiveelectrode was prepared using this slurry by the same method as inExample 1. A slurry containing negative active material was prepared bymixing artificial graphite as a negative active material andpolyvinylidene fluoride as a binder in the weight ratio of 90/10 in amixing solvent of NMP. The negative electrode was prepared by castingthe slurry containing the negative active material on a Cu foil. A 930mAh prismatic Li-ion cell was fabricated using the positive and thenegative electrodes. For the electrolyte, 1 M LiPF₆ solution of a mixedsolvent of ethylene carbonate/dimethyl carbonate in the volume ratio of1/1 was used.

EXAMPLE 16

[0106] A coin-typed half-cell was prepared by the same method as inExample 1, except that a 20 ml portion of the coating liquid of Example1 was added to 20 g of natural graphite. The total amount of Al and Pwas 2% by weight of the total weight of the total active materialweight.

EXAMPLE 17

[0107] A coin-typed half-cell was prepared by the same method as inExample 1, except that a 20 ml portion of the coating liquid of Example1 was added to 20 g of SnO₂. The total amount of Al and P was 2% of thetotal active material weight.

EXAMPLE 18

[0108] A coin-typed half-cell was prepared by the same method as inExample 1, except that a 20 ml portion of the coating liquid of Example1 was added to 20 g of silicon (Si) active material. The total amount ofAl and P was 2% by weight of the total active material weight.

COMPARATIVE EXAMPLE 1

[0109] A coin-typed half-cell was prepared by the same method as inExample 1 except that LiCoO₂ with an average particle diameter of 10 μmwas used as the positive active material.

COMPARATIVE EXAMPLE 2

[0110] A coin-typed half-cell was prepared by the same method as inExample 1, except that LiCoO₂ with an average particle diameter of 5 μmwas used as the positive active material.

COMPARATIVE EXAMPLE 3

[0111] A coin-typed half-cell was prepared by the same method as inExample 1, except that aluminum nitrate (Al(NO₃)₃·9H₂O) was not added tothe coating liquid, and thus the active material coated with P₂O₅ on thesurface was prepared.

COMPARATIVE EXAMPLE 4

[0112] A coin-typed half-cell was prepared by the same method as inExample 1, except that LiMn₂O₄ with an average particle diameter of 13μm was used as the positive active material.

COMPARATIVE EXAMPLE 5

[0113] A coin-typed half-cell was prepared by the same method as inExample 1 except that LiNi_(0.8)Mn_(0.2)O₂with an average particlediameter of 10 μm was used as the positive active material.

COMPARATIVE EXAMPLE 6

[0114] A coin-typed half-cell was prepared by the same method as inExample 1 except that LiNi_(0.9)Co_(0.1)Sr_(0.002)O₂ with an averageparticle diameter of 13 μm was used as the positive active material.

COMPARATIVE EXAMPLE 7

[0115] A coin-typed half-cell was prepared by the same method as inExample 1, except that Li_(1.03)Ni_(0.69)Mn_(0.19)Co_(0.1)Al_(0.07)Mg_(0.07)O₂with an average particle diameterof 13 μm was used as the positive active material.

[0116] Evaluation of Electrochemical Characteristics

[0117] Charge-discharge characteristics of the coin-typed half-cell ofExample 1 at 0.2C, 0.5C and 1C rates in the voltage range of 4.3V to2.75V are shown in FIG. 1. For comparison, the characteristics of thecell of Comparative Example 1 are also shown. As seen in FIG. 1, theinitial capacity of the cell of Comparative Example 1 is much smaller ata high rate (1 C) than those at low rates (0.2C and 0.5C). However, theinitial capacity of the cell of Example 1 is very high (152 mAh/g) evenat the high rate. This value is close to those at low rates.

[0118]FIG. 2 shows capacity characteristics for cycling at IC rate ofthe half-cells according to Example 1 and Comparative Example 1 in thevoltage range of 4.3V to 2.75V. In the Example 1, more than 99% ofinitial capacity was maintained after 30 charge-discharge cycles. Incontrast, the capacity of the cell of Comparative Example 1 decreasedsharply after 30 charge-discharge cycles. In addition, the averagedischarge voltage of the cell of Comparative Example 1 decreasedsignificantly with repeated cycling while the average value for Example1 showed a negligible change.

[0119]FIG. 3A shows the charge-discharge characteristics of a half-cellaccording to Example 5 at 0.2C, 0.5C and 1C rates in the voltage rangeof 4.3V to 2.75V. In order to confirm reproducibility ofcharge-discharge characteristics, powder of the positive active materialof Example 5 was synthesized in a large scale (1.5 kg batch size) andabout thirty cells were made to evaluate the charge-dischargecharacteristics. FIG. 3A represents the average values. FIG. 3B is anexpanded view of a portion of FIG. 3A. FIG. 3A shows that the initialcapacity at 1C rate was 150-152 mAh/g and the average voltage was about3.91V, which is similar to those at a low rate (0.2C). One noticeableobservation was that the discharge curve at 1C rate gradually approachesthat at 0.2C rate as the charge-discharge cycling at 1C rate proceeds.This means that there is a slight improvement rather than deteriorationof discharge capacity due to a decrease in internal resistance withcycling.

[0120]FIG. 4 shows the results of charge-discharge cycling of the cellof Example 2 at 0.2C, 0.5C and 1C rates in the voltage range of 4.3V to2.75V. On repeated charge-discharge cycling, the average voltages at a1C rate approach those at 0.2C rate. This observation is also similar tothose of Example 12 as shown in FIG. 5. This observation indicates thatthe coating technique of the present invention is effective in improvingthe cell performance for Ni-based compounds as well as for Co-basedcompounds.

[0121] Discharge capacities of FIG. 1, FIG. 3A, FIG. 4 and FIG. 5 aresummarized as shown in Table 1. TABLE 1 discharge capacities at variousC-rate rates (unit: mAh/g) C-rate Com. Ex. 1 Ex. 1 Ex. 2 Ex. 5 Ex. 120.2C 154 159 159 161 170 0.5C 151 156 156 159 161 1C 143 152 152 152 146

[0122] In order to evaluate the effects of heat-treatment temperatureand time on the electrochemical characteristics of cells, FIG. 6 showsthe results of charge-discharge cycling of the cells of Examples 5 to 7at 1C rate in the voltage range of 4.3V to 2.75V. The cells containingheat-treated positive active material powder at various heat-treatmenttemperatures and times showed improved high-rate characteristics overthose of the cell containing the conventional uncoated LiCoO₂(Comparative Example 1).

[0123]FIG. 7 shows the results of charge-discharge cycling of cells ofExample 1 and Comparative Example 1 at 0.2C, 0.5C and 1 C rates in thevoltage range of 4.6V to 2.75V instead of 4.3V to 2.75V. ComparativeExample 1 shows that as the discharge rate increases, the dischargecurve abruptly deteriorates and high cell polarization is indicated. Thecapacity at 1C rate after 30 cycles was less than 70% of the initialcapacity. On the contrary, the cell of Example 1 showed remarkablyimproved 1C-rate characteristics; i.e., high discharge voltage andimproved cycle life even after charging at 4.6V.

[0124]FIG. 8 shows the results of charge-discharge cycling of cells ofExample 1 and Comparative Example 1 at 0.2C, 0.5C and 1C rates in thevoltage range of 4.95V to 2.75V. The cell of Comparative Example 1 didnot show any measurable discharge capacity when it was charged at 4.95V,while the cell of Example 1 showed excellent discharge characteristicsafter charging at 4.95V, similar to the case of 4.6V charging.

[0125] As explained above, electrochemical characteristics of the cellcontaining positive active material of the present invention areexcellent because the solid-solution compound including Al and P, andthe AlPO_(k) (k is 2 to 4) compound which is formed on the surface ofthe lithiated intercalation compound probably improve conductivity oflithium ions and reduce surface polarization at high rates.

[0126] In order to verify the effect of the surface-treatment layer onthe performance of the cell, cycle life characteristics at 1C rate ofthe cells of Example 1 containing the active material that has thesurface-treatment layer comprising the solid-solution compound includingAl and P on the surface as well as the AlPO_(k) (k is 2 to 4) compoundand the cell of Comparative Example 3 containing the active materialconsisting of a P-containing layer derived from P₂O₅, are shown in FIG.9. Cycle life characteristics were measured by varying charge-dischargerate from 0.2C to 0.5C and 1C. As shown in FIG. 9, the cell capacity ofComparative Example 3 decreases rapidly with cycling, while the cellcapacity of Example 1 is maintained constant with cycling at the initialvalue.

[0127]FIG. 10 shows discharge voltage curves of a 930 mAh prismaticLi-ion cell prepared in Example 15 at discharge rates of 0.5C, 1C and 2Cin the voltage range of 4.2V to 2.75V. As shown in FIG. 10, thedischarge capacity of the cell at 2C rate was more than 95% of that at0.5C rate. Therefore, Li-ion cells comprising the active materialprepared according to the present invention have excellent cellperformance, similar to those of the coin-type half-cells.

[0128] In order to evaluate the relationship between tap density andcapacity of the active material of the present invention, electrodesprepared in Example 1 and Comparative Example 1 were cut into 4×4 cm²pieces and then the amount of active material was analyzed. The amountsfound were 150 mg in Example 1 and 120 mg in Comparative Example 1.Table 2 below shows electrode density, tap density and measured capacityof the active material. From Table 2, electrode density and tap densityof the active material of Example 1 were larger than those ofComparative Example 1, and the specific capacity of Example 1 is higherthan that of Comparative Example 1. TABLE 2 Electrode Tap SpecificSpecific Cell density density capacity at capacity at capacity (g/cm³)(g/cc) 0.2C (mAh/g) 0.5C (mAh/g) (mAh) Ex. 1 3.79 2.5 160 150 24 Com.3.42 2.1 160 143 19.2 Ex. 1

[0129] Hereinafter, the structure and components of thesurface-treatment layer will be explained.

[0130] Analysis of Structure and Components of Surface-Treatment Layer

[0131] Active material prepared according to Example 1 of the presentinvention has a surface-treatment layer comprising a solid-solutioncompound including Al and P and an AlPO_(k) (k is 2 to 4) compound onthe surface of the active material. In order to confirm the presence ofthe surface-treatment layer, elemental mapping was performed on thesurface of the cross-section of a grain of the surface-treated activematerial using STEM (Scanning Transmission Electron Microscopy). Theresults are shown in FIGS. 11A to 11E. FIG. 11A is a STEM photograph ofactive material in a bright field, and FIGS. 11B to 11E are STEMphotographs showing distribution of Co, Al, P and O respectively. Asshown in FIGS. 11B to 11E, Co, Al, P and O are all found in the surfaceportion of the particle, suggesting the existence of the solid-solutioncompound and the AlPO_(k) (k is 2 to 4) compound.

[0132] In order to analyze components of the surface-treatment layerformed on the surface of the active material prepared according toExample 1, an analysis for Al, O, Co, and P was carried out using Augerspectroscopy. FIG. 12 shows the result from the surface to a depth ofabout 380 Å. FIG. 12 shows that a layer of the solid-solution compoundincluding Al and P and another layer of the AlPO_(k) (k is 2 to 4)compound were formed from the surface to a depth of about 230 Å, andCoO₂ (possibly Li_(1-x)CoO₂ where x is greater than or equal to 0.5) wasformed further inside.

[0133] In order to estimate distribution of various elements through thebulk of the particle, Electron Probe Micro Analysis (EPMA) for Co, Al,and P was performed by line scanning across the cross-section of aparticle grain of the active material prepared in Example 1. FIGS. 13Aand 13B show the results. As shown in FIG. 13B, the presence of Al and Pwas found only in the surface layer of the particle at less than 1 μm indepth. These results indicate that the solid-solution compoundsincluding Al and P and the AlPO_(k) (k is 2 to 4) compound in thesurface-treatment layer did not diffuse further into the bulk of theactive material.

[0134]FIG. 14 shows the results of analysis for the solid-solutioncompound including Al and P and AlPO_(k) (k is 2 to 4) compound ofsurface-treatment layer of the active material prepared in Example 1using X-ray photoelectron spectroscopy (XPS). From FIG. 14, it can beconfirmed that peak positions of O and P in the surface-treated activematerial agrees well with that of P₂O₅, which indicates that a doublebond of P═O exists in the solid-solution compound and the AlPO_(k)compound. However, electrochemical characteristics of the solid-solutioncompound and the AlPO_(k) (k is 2 to 4) compound are not identical tothose of the P₂O₅ compound. For example, the capacity of the cell ofComparative Example 3 comprising active material coated with P₂O₅ on thesurface deteriorates rapidly with high-rate (1C) cycling, while the cellof Example 1 containing the active material comprising thesolid-solution compound and the AlPO_(k) (k is 2 to 4) compoundmaintains good capacity and average voltage on cycling both at a highrate as well as at a low rate (see FIG. 9). This is probably because,although the solid-solution compound and the AlPO_(k) compound ofExample 1 and the P₂O₅ compound have a double bond in the surface layerof the active material, the P₂O₅ compound is different from thecompounds in the surface-treatment layer in terms of effect on themobility of lithium ions. That is, the solid-solution compound and theAlPO_(k) compound that have a double bond probably promote the mobilityof lithium ions so that the capacity at the high rate can be maintainedat a high level.

[0135] In order to verify the effect of the AlPO_(k) compound upon themobility of lithium ions, oxidation and reduction peaks of cyclicvoltammograms of the cells of Comparative Example 1 and Example 5 werestudied. The cyclic voltammograms were measured in the voltage range of3V to 4.4V at a scanning rate of 0.02 mV/sec. Lithium metal was used asthe reference electrode in the cell. FIGS. 15A and 15B show the results.The widths of oxidation/reduction peaks in the cyclic voltammogram ofExample 5 are significantly smaller than those of Comparative Example 1,indicating that the electrode reaction rate is improved, and therefore,the mobility of lithium ions is also improved by the surface layer.

[0136] The fact that the mobility of lithium ions in the compound formedon the surface of the present positive active material is high wasconfirmed by the measurement of diffusion coefficient of lithium ions asshown in FIG. 16. The diffusion coefficient of lithium ions for Example5 of the present invention is five times as high as that of ComparativeExample 1.

[0137] In addition, the cyclic voltammograms of FIG. 15A for ComparativeExample 1 indicate that a phase transition occurs from a hexagonal phaseto a monoclinic phase and then it returns to the hexagonal phase atabout 4.1 to 4.25V. On the contrary, cyclic voltammograms of Example 5(FIG. 15B) have no peak that is assumed to be related to this phasetransition.

[0138] The reason that positive active material of the present inventiondoes not show a peak that is assumed to be related to the phasetransition in the cyclic voltammogram is because the c-axis, whichaffects volume expansion during charging, hardly changes. Changes oflattice constants (a-axis and c-axis) of active materials preparedaccording to Example 5 and Comparative Example 1 during charge-dischargein the voltage range of 4.6V to 4.25V were measured. FIGS. 17A and 17Bshow the results. Active material of Comparative Example 1 shows phasetransitions from hexagonal (H) to monoclinic (M) and back to hexagonal(H) phase in the voltage range of 4.1 to 4.25V during charge-discharge.The change of the c-axis lattice constant was larger than that of thea-axis. When these anisotropic contraction and expansion are more than0.2% of the elasticity limit of the active material, micro-cracksdevelop in the particles so that the particles break down to smallerparticles causing the decrease of the cell capacity. As indicated inFIG. 17A, the active material of Comparative Example 1 had contractionand expansion of more than 1% in the c-axis so that micro-cracksoccurred in the particles and thus the cell capacity decreased sharplyas the discharge rate increased (see FIGS. 1 and 7). On the contrary,the cell of Example 5 showed a significantly reduced variation oflattice constant of the c-axis (FIG. 17A) so that the cell capacity wasmaintained high at the high rates of discharge (see FIGS. 1, 7 and 8).

[0139] Evaluation of Thermal Stability

[0140] In order to evaluate thermal stability of the positive activematerial prepared according to Example 5 of the present invention andComparative Example 1, DSC analysis was performed as follows. The coincells of Example 5 and Comparative Example 1 were charged using avoltage cut-off at 4.3V. About 10 mg of the positive active materialsfrom charged electrodes of each cell were collected. DSC analyses werecarried out in sealed aluminum cans using a 910 DSC (TA Instrumentcompany) equipment by scanning temperatures from 25 to 300° C. at therate of 3° C./min. The results are shown in FIG. 18.

[0141] As shown in the FIG. 18, Comparative Example 1 (notsurface-treated LiCoO₂) showed a large exothermic peak in thetemperature range of 180 to 220° C. as a result of 02 release from thebreakage of Co-O bonds of charged Li_(1-x)CoO₂ followed by theexothermic reaction of the oxygen with electrolyte. This phenomenon isknown as the cause of the safety problem. However, in the case ofExample 5, the exothermic peak in the DSC was reduced to a negligiblesize, strongly suggesting that the thermal stability, and therefore thesafety of the batteries using the active material of Example 5 will bemuch better than that of Comparative Example 1.

[0142] In order to evaluate the thermal stability of the active materialas a function of the heat-treatment temperature and time, the DSCmeasurements of the charged electrodes at the cut-off voltage of 4.3Vfor the cells of Examples 6 and 7 were carried out as shown in FIG. 19.The open circuit voltage (OCV) of the cell disassembly was maintained atover 4.295V. As shown in FIG. 19, the electrode samples from the cellsof Examples 5 to 7 showed excellent thermal stability.

[0143] After overcharging cells of Examples 1 and 8 at 4.6V, thermalproperties of the charged cells were measured by the same method as inthe case of 4.3V-charge. Results are shown in FIGS. 20 and 21. The cellsof Examples 1 and 8 which were charged at 4.6V showed negligibleexothermic heat evolution while those of Comparative Examples 1 and 2which are charged at the same voltage showed large exothermic peaksindicating excellent thermal stability of the positive active materialof the present invention.

[0144] If LiCoO₂ is overcharged at voltages over 4.93V, it turns to aCdl₂-type hexagonal structure that does not have Li. This structure hasthe least unstable state of the active material and it reacts rapidlywith electrolyte at high temperature, releasing oxygen. Afterovercharging the cell of Example 8 at 5V, DSC measurement was carriedout. The result is shown in FIG. 22. The DSC measurements of the cellsof Examples 1 and 9 were also carried out. The results are shown in FIG.23. As shown in FIGS. 22 and 23, in the case of overcharging to 5V, thecells of Comparative Examples 1 and 2 that do not have thesurface-treatment layer show a low starting temperature (T_(s)) of theexothermic reaction and a relatively large exothermic heat, and multipleexothermic peaks were observed. Although active materials afterovercharging the cells of Examples 1, 8 and 9, which contained positiveactive material coated with the solid-solution compound and the AlPO_(k)(k is 2 to 4) compound, at 5V showed exothermic peaks unlike the case ofthe 4.3V- and 4.6V-charge, the starting temperature of the exothermicreaction was over 235° C. which is more than 40° C. higher than that ofconventional LiCoO₂ (Comparative Examples 1 and 2) without the surfacetreatment.

[0145] The coated active material, LiCoO₂, according to the presentinvention, has comparable or better thermal stability andcharacteristics (Examples 1 and 9), as shown in FIG. 23, than uncoatedLiMn₂O₄ (Comparative Example 4) active material which is known for itsgood thermal stability and characteristics among positive materials usedin commercially available lithium secondary battery cells.

[0146] In order to evaluate thermal stability of positive activematerials of Example 10 and Comparative Example 4 (not surface-treatedLiMn₂O₄), DSC measurements were carried out using a similar technique asdescribed above. The results are shown in FIG. 24. Even without thesurface-treatment, LiMn₂O₄ is known as the most thermally stable ofpositive active materials. The exothermic heat of active materialcontaining the surface-treatment of the AlPO_(k) compound is remarkablyreduced from that of the untreated material indicating that the thermalstability is remarkably improved by the surface-treatment.

[0147]FIG. 25 shows the results of a DSC evaluation of the thermalstability of positive active materials of Examples 12 and 13 andComparative Example 5 (not surface-treated LiNi_(0.8)Mn_(0.2)O₂). Thecells of Examples 12 and 13 and Comparative Example 5 were overchargedat 4.6V. Fully charged positive electrode samples were taken from thecells. About 10 mg of the active material was removed from the electrodesamples. The test material was sealed in an aluminum can and DSCthermograms were obtained using 910 DSC (TA Instrument company)equipment by scanning temperatures from 100 to 300° C. at the rate of 3°C./min. As shown in FIG. 25, the values of the exothermic heat of thesurface-treated LiNi_(0.8)Mn_(0.2)O₂ active material according toExamples 12 and 13 decreased to about {fraction (1/30)} of the value ofthe active material of Comparative Example 5 that was notsurface-treated. Therefore, the thermal stability of the nickelmanganese-based active material is improved significantly, similar tothe cases of the cobalt-based and manganese-based active materials.

[0148] Twenty 930 mAh prismatic Li-ion cells comprising the positiveactive materials prepared in Examples 1, 8 and 9, and ComparativeExamples 1 (conventional, not surface-treated, LiCoO₂) and 4(conventional, not surface-treated, LiMn₂O₄) were safety tested for thecategories of combustion, heat exposure and overcharge. The sampleLi-ion cells were prepared by the same method as in Example 15. Thecombustion test was performed by heating cells using a burner, andmeasuring the percentage of the cells that underwent an explosion. Theheat exposure test was performed by exposing the cells to 150° C.measuring times before an explosion time was measured. The overchargetest was carried out by observing the percentages of the sample cellsthat were overcharged at 1C rate. These results are presented in Table3. TABLE 3 Com. Com. Ex. 1 Ex. 4 Ex. 1 Ex. 8 Ex. 9 Combustion test 100%50% 0% 0% 0% (Explosion percentage) Heat exposure test 5 min. 20 min. 90min. 95 min. 95 min. (Average explosion Time) Overcharge test 90% 30% 0%0% 0% (Leak percentage)

[0149] For the above safety tests for 930 mAh Li-ion cells comprisingpositive active materials of Examples 1, 8 and 9 and ComparativeExamples 1 and 4, the overcharge test was carried out at three differentcharge rates, 1C, 2C, and 3C, as shown in Table 4. Five cells weretested for each charge rate. TABLE 4 C-rate Com. Ex. 1 Com. Ex. 4 Ex. 1Ex. 8 Ex. 9 1C 5L5 5L0 5L0 5L0 5L0 2C 5L5 5L5 3L0, 2L4 3L0, 2L4 3L0, 2L43C 5L5 5L5 4L3, 1L4 4L3, 1L4 4L3, 1L4

[0150] The number preceding “L” indicates the number of tested cells.

[0151] The results of the safety test were rated as follows:

[0152] L0: good, L1: leakage, L2: flash, L2: flame, L3: smoke, L4:ignition, L5: explosion.

[0153] The temperature of the prismatic Li-ion cell of Example 15 wastested for the increased charge voltage to 12V as shown in FIG. 26.Generally, when the cell voltage dropped to 0V during the high-voltageovercharge, the cell explodes and the cell temperature increasesabruptly. However, the cell of Example 15 did not show the increase oftemperature over 100° C. when the charge voltage drops from 12V to 0V asshown in FIG. 26. Therefore, the cells prepared using thesurface-treated active material of the present invention show excellentsafety.

[0154] The active material containing a surface-treatment layercomprising the MXO_(k) (k is 2 to 4) compound of the present inventionshows excellent structural stability and high average discharge voltagesboth at high and low rates, excellent cycle life characteristics, andgood capacity. Its excellent thermal stability improves the safety ofthe cells in various categories including combustion, heat exposure, andovercharge tests. In addition, the process of the present invention usesa water-based coating liquid giving a great low cost benefit over asimilar process using an organic solvent-based solution. Since theprocess is performed at a lower temperature and in a shorter processtime than the conventional process using an organic solvent,productivity is expected to be improved in large-scale production.

[0155] The foregoing is considered illustrative only of the principlesof the invention. Further, since numerous modifications and changes willreadily occur to those skilled in the art, it is not desired to limitthe invention to the exact construction and operation shown anddescribed. Accordingly, all suitable modifications and equivalents maybe resorted to that fall within the scope of the invention and theappended claims.

What is claimed is:
 1. An active material for a battery having asurface-treatment layer, comprising a compound of the formula (1):MXO_(k)  (1) wherein M is at least one element selected from the groupconsisting of an alkali metal, an alkaline earth metal, a group 13element, a group 14 element, a transition metal, and a rare-earthelement, X is an element that is capable of forming a double bond withoxygen, and k is a numerical value in the range of 2 to
 4. 2. The activematerial of claim 1, wherein the element M is selected from the groupconsisting of Na, K, Mg, Ca, Sr, Ni, Co, Si, Ti, B, Al, Sn, Mn, Cr, Fe,V, Zr, and a combination thereof.
 3. The active material of claim 1,wherein the element X is selected from the group consisting of P, S, W,and a combination thereof.
 4. The active material of claim 1, wherein anamount of the element M is 0.1 to 15% by weight of the active material.5. The active material of claim 1, wherein an amount of the element M is0.1 to 6% by weight of the active material.
 6. The active material ofclaim 1, wherein an amount of the element X is 0.1 to 15% by weight ofthe active material.
 7. The active material of claim 1, wherein anamount of the element X is 0.1 to 6% by weight of the active material.8. The active material of claim 1, wherein a thickness of thesurface-treatment layer is 0.01 to 2 μm.
 9. The active material of claim1, wherein a tap density of the active material is 1 to 3 g/cc.
 10. Theactive material of claim 1, wherein the active material is anelectrochemically reversibly oxidizable and reducible material.
 11. Theactive material of claim 10, wherein the electrochemically oxidizableand reducible material is selected from the group consisting of a metal,a lithium-containing alloy, a sulfur-based compound, a compound that canreversibly form a lithium-containing compound by a reaction with lithiumions, and a material that can reversibly intercalate/deintercalatelithium ions.
 12. The active material of claim 11, wherein the materialthat can reversibly intercalate/deintercalate lithium ions is one oflithium-containing metal oxide, a lithium containing chalcogenidecompound, and a carbon-based material.
 13. The active material asrecited in claim 1, wherein the active material is used in at least oneof a positive electrode and a negative electrode of the battery.
 14. Theactive material as recited in claim 1, wherein the battery is one of amanganese battery, an alkaline battery, a mercury battery, a silveroxide battery, a lead-acid storage battery, a nickel metal hydridebattery, a nickel-cadmium battery, a lithium metal battery, a lithiumion battery, a lithium polymer battery and a lithium-sulfur battery. 15.An active material for a battery, comprising: a lithiated intercalationcompound selected from the group consisting of a lithium-containingmetal oxide and a lithium-containing chalcogenide compound; and asurface-treatment layer is formed on a surface of the lithiatedintercalation compound and has a solid-solution compound includingsurface treating elements M and X, and a compound of the formula (1):MXO_(k)  (1) wherein M is at least one element selected from the groupconsisting of an alkali metal, an alkaline earth metal, a group 13element, a group 14 element, a transition metal, and a rare-earthelement, X is an element which is capable of forming a double bond withoxygen, and k is a numerical value in the range of 2 to
 4. 16. Theactive material of claim 15, wherein the element M is selected from thegroup consisting of Na, K, Mg, Ca, Sr, Ni, Co, Si, Ti, B, Al, Sn, Mn,Cr, Fe, V, Zr, and a combination thereof.
 17. The active material ofclaim 15, wherein the element X is selected from the group consisting ofP, S, W, and a combination thereof.
 18. The active material of claim 15,wherein an amount of the element M is 0.1 to 15% by weight of the activematerial.
 19. The active material of claim 15, wherein an amount of theelement M is 0.1 to 6% be weight of the active material.
 20. The activematerial of claim 15, wherein an amount of the element X is 0.1 to 15%by weight of the active material.
 21. The active material of claim 15,wherein an amount of the element X is 0.1 to 6% by weight of the activematerial.
 22. The active material of claim 15, wherein a thickness ofthe surface-treatment layer is 0.01 to 2 μm.
 23. The active material ofclaim 15, wherein a tap density of the active material is 1 to 3 g/cc.24. The active material of claim 15, wherein a concentration of theelements M and X decreases gradually from a surface to a center of aparticle grain of the active material.
 25. The active material of claim15, wherein the lithiated intercalation compound has one of amonoclinic, hexagonal and a cubic structure as a basic structure. 26.The active material of claim 15, wherein the lithiated intercalationcompound is at least one selected from the group consisting of a lithiumcompound with the following formulas (2) to (14): Li_(x)Mn_(1−y)M′_(y)A₂(2) Li_(x)Mn_(1−y)M′_(y)O_(2−z)B_(z) (3) Li_(x)Mn₂O_(4−z)B_(z) (4)Li_(x)Mn_(2−y)M′_(y)A₄ (5) Li_(x)Co_(1−y)M′_(y)A₂ (6)Li_(x)Co_(1−y)M′_(y)O_(2−z)B_(z) (7) Li_(x)Ni_(1−y)M′_(y)A₂ (8)Li_(x)Ni_(1−y)M′_(y)O_(2−z)B_(z) (9) Li_(x)Ni_(1−y)Co_(y)O_(2−z)B_(z)(10) Li_(x)Ni_(1−y−z)Co_(y)M′_(z)A_(a) (11)Li_(x)Ni_(1−y−z)Co_(y)M′_(z)O_(2−a)B_(a) (12)Li_(x)Ni_(1−y−z)Mn_(y)M′_(z)A_(a) (13)Li_(x)Ni_(1−y−z)Mn_(y)M′_(z)O_(2−a)B_(a) (14)

wherein 0.95≦x≦1.1,0≦y≦0.5,0≦z≦0.5, and 0<α<2; M′ is at least oneelement selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe,Mg, Sr, V, and a rare-earth element; A is at least one element selectedfrom the group consisting of O, F, S and P; and B is at least oneelement selected from the group consisting of F, S and P.
 27. The activematerial of claim 26, wherein an average particle size of the lithiatedintercalation compound is 1 to 20 μm.
 28. An active material for abattery, comprising: a lithiated intercalation compound selected fromthe group consisting of a lithium-containing metal oxide and alithium-containing chalcogenide compound; and a surface-treatment layerformed on a surface of the lithiated intercalation compound and has asolid-solution compound including Al and P, and an AlPO_(k) (k is 2 to4) compound.
 29. The active material of claim 28, wherein an amount ofthe element Al is 0.1 to 15% by weight of the active material.
 30. Theactive material of claim 28, wherein an amount of the element Al is 0.1to 6% by weight of the active material.
 31. The active material of claim28, wherein an amount of the element P is 0.1 to 15% by weight of theactive material.
 32. The active material of claim 28, wherein an amountof the element P is 0.1 to 6% by weight of the active material.
 33. Theactive material of claim 28, wherein a concentration of the elements Mand X decreases gradually from a surface to a center of a particle grainof the active material.
 34. The active material of claim 28, wherein thelithiated intercalation compound has one of a monoclinic, hexagonal anda cubic structure as a basic structure.
 35. The active material of claim28, wherein the lithiated intercalation compound is at least oneselected from the group consisting of a lithium compound with thefollowing formulas (2) to (14): Li_(x)Mn_(1−y)M′_(y)A₂ (2)Li_(x)Mn_(1−y)M′_(y)O_(2−z)B_(z) (3) Li_(x)Mn₂O_(4−z)B_(z) (4)Li_(x)Mn_(2−y)M′_(y)A₄ (5) Li_(x)Co_(1−y)M′_(y)A₂ (6)Li_(x)Co_(1−y)M′_(y)O_(2−z)B_(z) (7) Li_(x)Ni_(1−y)M′_(y)A₂ (8)Li_(x)Ni_(1−y)M′_(y)O_(2−z)B_(z) (9) Li_(x)Ni_(1−y)Co_(y)O_(2−z)B_(z)(10) Li_(x)Ni_(1−y−z)Co_(y)M′_(z)A_(a) (11)Li_(x)Ni_(1−y−z)Co_(y)M′_(z)O_(2−a)B_(a) (12)Li_(x)Ni_(1−y−z)Mn_(y)M′_(z)A_(a) (13)Li_(x)Ni_(1−y−z)Mn_(y)M′_(z)O_(2−a)B_(a) (14)

wherein 0.95≦x≦1.1,0≦y≦0.5,0≦z≦0.5, and 0<α<2; M′ is at least oneelement selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe,Mg, Sr, V, and a rare-earth element; A is at least one element selectedfrom the group consisting of O, F, S and P; and B is at least oneelement selected from the group consisting of F, S and P.
 36. The activematerial of claim 35, wherein an average particle size of the lithiatedintercalation compound is 1 to 20 μm.
 37. The active material of claim28, wherein a tap density of the active material is 1 to 3 g/cc.
 38. Amethod of preparing an active material for a battery, comprising:preparing a coating liquid by adding a compound including an elementthat is capable of forming a double bond with oxygen of lithium metaloxide, and a metal compound comprising at least one element from thegroup consisting of an alkali metal, an alkaline earth metal, a group 13element, a group 14 element, a transition metal, and a rare-earthelement, to water; adding active material to the coating liquid to coatthe active material; and heat-treating the coated active material toform a surface-treatment layer comprising a compound having the formula(1): MXO_(k)  (1) wherein M is at least one element selected from thegroup consisting of an alkali metal, an alkaline earth metal, a group 13element, a group 14 element, a transition metal, and a rare-earthelement, X is an element that is capable of forming a double bond withoxygen, and k is a numerical value in the range of 2 to
 4. 39. Themethod of claim 38, wherein the element M is selected from the groupconsisting of Na, K, Mg, Ca, Sr, Ni, Co, Si, Ti, B, Al, Sn, Mn, Cr, Fe,V, Zr, and a combination thereof.
 40. The method of claim 38, whereinthe element X is selected from the group consisting of P, S, W, and acombination thereof.
 41. The method of claim 38, wherein an amount ofthe element M is 0.01 to 30% by weight of the coating liquid.
 42. Themethod of claim 38, wherein an amount of the element M is 0.01 to 20% byweight of the coating liquid.
 43. The method of claim 38, wherein anamount of the element X is 0.01 to 30% by weight of the coating liquid.44. The method of claim 38, wherein an amount of the element X is 0.01to 20% by weight of the coating liquid.
 45. The method of claim 38,wherein the active material is an electrochemically reversiblyoxidizable and reducible material.
 46. The method of claim 45, whereinthe electrochemically oxidizable and reducible material is selected fromthe group consisting of a metal, a lithium-containing alloy, asulfur-based compound, a compound that can reversibly form alithium-containing compound by a reaction with lithium ions, and amaterial that can reversibly intercalate/deintercalate lithium ions. 47.The active material of claim 46 wherein the material that can reversiblyintercalate/deintercalate lithium ions is one of lithium-containingmetal oxide, a lithium-containing chalcogenide compound, and acarbon-based material.
 48. The method of claim 38, wherein theheat-treating step is performed at 100 to 700° C.
 49. The method ofclaim 38, wherein the heat-treating step is performed at 100 to 500° C.50. The method of claim 38, further comprising, prior to the heatingstep, drying the coat formed on the active material.
 51. The method ofclaim 38, wherein the heat treating step is performed for 1 to 20 hours.52. A method of forming a battery, comprising: preparing a coatingliquid by adding a compound including an element that is capable offorming a double bond with oxygen of lithium metal oxide, and a metalcompound comprising at least one element from the group consisting of analkali metal, an alkaline earth metal, a group 13 element, a group 14element, a transition metal, and a rare-earth element, to water; addingactive material to the coating liquid to coat the active material; andheat-treating the coated active material to prepare an active materialhaving a surface-treatment layer comprising a compound having theformula (1): MXO_(k)  (1) wherein M is at least one element selectedfrom the group consisting of an alkali metal, an alkaline earth metal, agroup 13 element, a group 14 element, a transition metal, and arare-earth element, X is an element that is capable of forming a doublebond with oxygen, and k is a numerical value in the range of 2 to 4;coating a slurry comprising the active material with the surfacetreatment layer onto a current collector to prepare at least one of apositive and negative electrode; and fabricating a battery using theprepared one of the positive and negative electrodes and a correspondingother one of the positive and negative electrodes.
 53. The method ofclaim 52, wherein the active material is selected from electricallyreversibly oxidizable and reducible materials.
 54. The method of claim52, wherein the battery is selected from a manganese battery, analkaline battery, a mercury battery, a silver oxide battery, a lead-acidstorage battery, a nickel metal hydride battery, a nickel-cadmiumbattery, a lithium metal battery, a lithium ion battery, a lithiumpolymer battery and a lithium-sulfur battery.